In-situ synthesis of nano-lamellar Ni1.5CrCoFe0.5Mo0.1Nbx eutectic high-entropy alloy coatings by laser cladding: Alloy design and microstructure evolution

In this work, a novel Ni1.5CrCoFe0.5Mo0.1Nb0.68 eutectic HEA coating with nano-lamellar microstructure was successfully designed by binary eutectic compositions strategy, thermodynamic calculation and simple experimental approach. Compared with the predicted eutectic compositions, the actual eutectic composition had a higher Nb content due to the increased solid solubility of Nb in the FCC phase. The Ni1.5CrCoFe0.5Mo0.1Nbx (x = 0.55 hypoeutectic, 0.68 eutectic, 0.8 hypereutectic) HEA coatings were prepared by laser cladding, which were composed of FCC phase with lattice parameter of a = ~3.59 Å and Laves phase with lattice parameter of a = b = ~4.82 Å and c = ~7.80 Å. The FCC phase was enriched in Ni, Cr, Co and Fe elements, while the Laves phase was enriched in Nb and Mo elements. Typical eutectic lamellar and hypoeutectic/hypereutectic microstructures were obtained at the central regions, while the columnar crystals formed at the interfacial regions. The average micro-hardness of Nb0.55, Nb0.68 and Nb0.8 coatings were calculated as ~573.5 HV, ~665.8 HV and ~715.6 HV respectively. From x = 0.55 to x = 0.8, the wear resistance of Ni1.5CrCoFe0.5Mo0.1Nbx HEA coatings are increased, and the dominant wear mechanism of the HEA coatings transforms from adhesive wear to abrasive wear.